This invention relates to a semiconductor device for a tuner for use in tuners of television receivers (hereinafter referred to as TV tuners") for satellite broadcasting, having low noise, low third order distortion and moreover, low power consumption.
FIG. 11 of the accompanying drawings shows a block diagram of a prior art TV tuner circuit which is an indoor TV tuner (so-called "BS tuner") for receiving satellite broadcasting, by way of example. In the drawing, reference numeral 1001 denotes an input terminal from an outdoor unit, 1002 is a band-pass filter, 1003 is a broad-band RF amplifier, 1004 is a variable attenuator, 1005 is a mixer circuit, 1006 a variable gain IF amplifier, 1007 is a band-pass filter, 1008 is an FM demodulator, 1009 is a video signal output terminal, 1010 is a buffer circuit for local oscillation signals, and 1012 is a phase-locked loop (PLL) circuit for stabilizing an oscillation frequency.
The intensity of the input signal varies greatly in the TV tuner circuit depending on the condition of use. In the case of ordinary home use, the signal intensity is from -60 dBm to -40 dBm but in the case of a community receiving system, an input of about 0 dBm is sometimes inputted. In the case of the excessive input, third order distortion of mutually different channel signals enters the band and leads to cross modulation. Therefore, it is necessary to damp the excessive input signal and to prevent distortion of the video signals by the use of a variable attenuator and a variable gain amplifier. Also, the TV tuner must by all means secure a sufficient third order distortion suppression ratio for any input signal intensity.
In the prior art example shown in FIG. 11, a variable attenuator comprising the RF amplifier 1003 and a PIN diode and the variable gain IF amplifier 1006 are employed as the variable attenuator of the input circuit portion in order to prevent excessive input. If the input signal is weak in this case, the damping factor of the variable attenuator 1004 is set to 0 dB and the broad band RF amplifier 1003 amplifies the signal. In this instance, the broad band RF amplifier 1003 must have a high noise factor. If the excessive input signal is inputted, on the other hand, the damping factor of the variable attenuator 1004 is set to -40 dB so as to prevent the excessive signal from being inputted to the mixer circuit 1005 but in this case, too, the excessive input signal is inputted to the RF amplifier 1003. For this reason, the RF amplifier 1003 must simultaneously possess excellent third order distortion characteristics and excellent cross modulation characteristics. In practice, however, the RF amplifier 1003 is used at an operation point at which current consumption is great, or a greater importance is set to only one of these characteristics, in order to simultaneously satisfy the low noise characteristics and the low third order distortion characteristics. These problems impede higher performance and lower power consumption of the tuner.
The prior art includes also a circuit which uses a variable gain RF amplifier using a dual gate FET shown in FIG. 12A to replace the variable attenuator 1004 comprising the PIN diode of FIG. 11. FIG. 12A shows a circuit example of the variable gain RF amplifier, and FIG. 12B shows a third order distortion suppression ratio (dBc, -20 dBm input) for the gain of this circuit and its consumed current (mA).
When this variable gain RF amplifier using the dual gate FET is employed, a wide gain variable width of 30 dB to 40 dB can be secured even in a single stage amplification circuit.
However, there are, on the other hand, various other problems. For example, the distortion characteristics cannot be improved even if the gain is lowered at the time of application of the excessive input signal because the input impedance changes with the change of the damping factor, because the third order distortion changes in a complicated way with the decrease of the gain as shown in FIG. 12B and moreover because the third order distortion suppression ratio deteriorates with the decrease of the gain within the range of gain of 0 to 20 dB. Since Ids of the FET greatly changes with the change of the gain, it is difficult to keep the D.C. bias of the output by resistance load. Degradation of the third order distortion characteristics when the gain of the dual gate FET is lowered occurs because Ids is contracted by the second gate and the operation point enters the non-saturation region of the FET.
Furthermore, the conventional tuner circuit is produced by integrating discrete devices on a printed substrate. Therefore, the number of man-hours for assembly cannot be reduced when the tuner circuits are mass-produced, and since there are a large number of points which need adjustment, there are inevitable limits to the improvement in efficiency of the assembly process of the TV tuners and their scale-down.
According to the variable gain amplifier and the mixer circuit system of the prior art, it has been difficult to simultaneously satisfy the low noise factor, low third order distortion characteristics and low power consumption required for the TV tuners, and the reduction of the number of man-hours for the assembly and scale-down of the tuners have been difficult, as well.